On a long trip to Mars, keep the astronaut/cosmonaut crews
busy with interesting variety of tasks to keep them occupied, challenged,
and vigorous enough to tire them out at the end of each shift
so they sleep well
Good food [prepared by themselves occasionally].
Interesting entertainment.
And a good command structure [Like on Mr. Shackleton's Antarctic
expeditions; or onboard atomic submarines] would be helpful too.

Keep the crews busy, but not too busy.
You don't want a mutiny against or disobedience or frictition
towards mission-control like what happened on Apollo 7, or on the last Skylab mission.

Jacob